The quadrupole mass filter of W. Paul et al described in U.S. Pat. No. 2,939,952, issued June 7, 1960, consists of four substantially parallel hyperbolic sheet electrodes (or cylindrical rods), symmetrically disposed about an axis. Opposite rods are electrically connected. On one pair of electrically connected oppositely disposed electrodes a dc voltage, U, and an ac voltage of amplitude, V, are placed. On the other pair of electrically connected oppositely disposed electrodes identical voltages, except having an electrical polarity opposite to the first pair, are placed. With proper settings of the dc voltage and the amplitude of the ac voltages, ions of a given charge-to-mass ratio have stable trajectories and oscillate about the axis whereby they do not collide with the electrodes; ions of other than the given charge-to-mass ratio are on unstable trajectories whereby they strike the electrodes. If ions are injected along the axis of the electrode structure, those with the given charge-to-mass ratio do not strike the electrodes and emerge from the opposite end of the electrode structure; however, ions with other than the given charge-to-mass ratio are accelerated in the transverse directions so that they collide with the electrodes and therefore do not emerge from the opposite end of the electrode structure. In this manner, the electrode structure functions as an ion "mass filter."
As noted in U.S. Pat. No. 3,129,327 to W. M. Brubaker of Apr. 14, 1964, an ion entering the electrode structure must pass through fringe fields near and beyond the end of the electrode structure. The ions must also pass through a similar fringe field in emerging from the opposite end of the electrode structure. As pointed out in the aforesaid patent of W. M. Brubaker, the ratio of the dc field strengths to ac field strength in the fringe fields is the same as in the electrode structure itself. Also, as disclosed in the aforesaid patent, an ion of the given charge-to-mass ratio, which is stable within the electrode structure proper, is on an unstable trajectory when it is in the fringe fields. Thus, although an ion would be stable within the electrode structure proper, it may not be received in the electrode structure proper due to its unstable trajectory in the fringe fields. This greatly reduces the transmission of ions of a given charge-to-mass ratio due to their rejection while within the fringe fields.
U.S. Pat. No. 3,129,327 further teaches that the ion trajectories can be stabilized on passage through the fringe fields provided that the ratio of the dc voltage (U) to the ac voltage amplitude (V) is reduced to a lower value than appropriate for use within the electrode structure proper. The aforesaid patent indicates several ways in which this can be accomplished in the case of quadrupole mass filters which have conventional metal electrodes. But the patent does not address itself to the broader problem of spatial separation of ac and dc fields emanating from the same metallic electrodes.
The copending patent application Ser. No. 346,250 teaches that separation of the high frequency ac fringe fields from the low frequency (including dc) fringe fields, is possible by placement of a tube or other appropriate geometrical configuration near an end or ends of a quadrupole mass filter wherein the tube is composed of a material which appears as a dielectric to the high frequency ac fringe fields and as a conductor to the low frequency (including dc) fringe fields. A required characteristic of such material is that the parameter 4.pi..sigma./.epsilon..omega. be much less than unity in value, where .sigma. is the dc electrical conductivity of the material and .epsilon. is the material's dielectric constant. The angular frequency, .omega. is equal to 2.pi.f where f is the frequency of the high-frequency ac fields.
Materials having the necessary physical characteristics exist and are readily available. Among such materials is a Nickel-zinc type ferrite manufactured by Stackpole Carbon Company of St. Mary's, Pa., known as "Cerramag C/12, " which has a volume resistivity at 25.degree.C. of about 3 .times. 10.sup.7 ohm-cm and a dielectric constant at 1.0MHz of about 10. When ions or electrons or both pass inside and along the length of a tube of such material, some of said ions or electrons or both strike the tube's walls producing effectively dc electrical currents within the material which may affect the dc potential of the tube. The dc potential of the tube can thus be changed through the IR drop of such currents, so that trajectories of ions which do not strike the tube's walls but pass on into the quadrupole mass filter are affected adversely. Another nickel-zinc ferrite manufactured by Stackpole Carbon Company which may be utilized in the invention, Cerramag C/11, has a resistivity of 2 .times. 10.sup.7 ohm-cm. A still further substance which has been tested and found operable is slate, the test sample having a resistivity of 1.0 .times. 10.sup.6 ohm-cm along two axes and 2.2 .times. 10.sup.6 ohm-cm along the third axis. The dielectric constant of slate is 6.0-7.5. The basic formula for the ferrites, Cerramag C/12 and Cerramag C/11, may be found in the patent of Zerbes U.S. Pat. No. 3,036,009 wherein other characteristics of the ferrite not important to the instant invention are discussed.
It is essential that the shield in the instant invention must appear to high-frequency ac fields as a dielectric and to low frequency, including direct current, fields as a conductor. Thus, for frequencies on the order of 1 megahertz, the resistivity must be substantially greater than 10.sup.5 ohm-cm. However, the materials utilized for the shielding effect are to be contrasted with good dielectrics which have resistivities of 10.sup.12 ohm-cm and higher. It is important that the resistivity of the material be not so high as to be unable effectively to conduct away current caused by ions or electrons which may strike the material during their transmission or receipt therein. Also, there are practical upper limits on the resistivity which basically relate to the sweep rate which may be utilized in the quadrupole mass filter. In this connection, it is to be understood that the substantially dc fields may be fields in a mass filter up to about 1000 Hz because this is about the limit of the sweep rate of the quadrupole mass filter. Thus, it is desirable that the material act like a conductor at a frequency up to 1000 Hz but as a dielectric at 1 million Hz. As a practical matter, materials having resistivities up to about 10.sup.8 ohm-cm are operable even at the maximum sweeping rate. By reducing the sweep rate and in many applications a sweep rate of about 10 Hz is all that is desirable, materials with resistivities up to about 10.sup.10 ohm-cm are operable. Still further, there are bona fide applications wherein resistivities up to about 10.sup.11 ohm-cm. However, a resistivity up to roughly 10.sup.8 ohm-cm offers improvement in every application of a quadrupole mass filter. From the foregoing it will be understood by those skilled in the art that the upper limit of resistivity of the shielding material varies depending upon the sweep rate desired in the mass filter, the configuration of the shield and the total ion current necessary for the shield to carry off.